Is it okay to put two 25KV OC-250 opto-couplers in series to get a 50KV opto-coupler?

We don’t recommend it. By connecting two OC-250s in series, the volts-per-mil stress between the LED and the diode in the second OC-250 in series is greater than 350V/mil. 350V/mil exceeds design parameters for the molding material used in the OC-250.